In a conventional sputter target assembly, the sputter target is attached to a non-magnetic backing plate. The backing plate holds the sputter target in a sputtering chamber and also provides structural support to the sputter target. In addition to this, the sputter target may contain cooling channels. During sputtering, water travels through these cooling channels to prevent over-heating of the sputter targets. Over-heating the sputter target can cause grain growth within the target and deflection of the target itself. Both grain growth and target deflection are disastrous to achieving uniform sputtering.
In view of these considerations, it is essential that sputter targets contain strong bonds with excellent thermal conductivity. Bonding techniques used to secure sputter targets include soldering, brazing, diffusion bonding, explosion bonding, mechanical fastening and epoxy bonding. Although commercial manufacturers use all of these techniques to manufacture sputter target assemblies, each of these techniques has certain disadvantages and none of the techniques provide a universal solution acceptable for all sputter target materials and configurations.
Brazing is the most common technique used to form a strong bond between a backing plate containing cooling channels and a sputter target. Unfortunately, this technique is not acceptable for sputter targets formed from aluminum or aluminum alloys, because the high temperatures associated with brazing increase the target's grain size to several millimeters. Similarly, the brazing of titanium targets to aluminum alloy backing plates often results in edge voids due to the large difference in the coefficients of thermal expansion.
Hartsough et al., in U.S. Pat. No. 5,985,115, describe the use of epoxy bonding to attach a channeled backing plate to a sputter target—the channeling of the backing plate can easily reduce the total bond area by fifty percent. Although this epoxy bonding technique is useful for smaller targets, it lacks the strength required for large sputter target assemblies. In the unlikely event that an epoxy bond delaminates during sputtering, it destroys both a wafer in progress and the electrostatic chuck supporting the wafer. In view of this large cost associated with an inadequate bond, chip manufacturers are uncomfortable relying upon an epoxy bond to secure a sputter target for 300 mm wafers to a channeled backing plate.
Other less common techniques for securing sputter targets include explosion and diffusion bonding. Explosion bonding avoids the detrimental grain growth associated with brazing low temperature melting point sputter targets such as aluminum and aluminum alloy targets. But explosion bonding tends to both deform and collapse cooling channels, particularly cooling channels constructed from aluminum alloys. As illustrated by Kordokus et al. in U.S. Pat. No. 5,803,342, diffusion bonding is useful for some sputter target assembly combinations. Unfortunately, diffusion bonding often requires excessive temperatures and results in detrimental grain growth. In addition to this, the compressive forces required for diffusion bonding can press a soft target material into the backing plate's cooling channels.
Ohhashi et al., in U.S. Pat. No. 5,693,203, disclose the use of solid state bonding to avoid the high pressure and temperatures typically required for diffusion bonding. This patent describes pressing a metal foil between a backing plate and a sputter target to form a solid state bond. Although this technique produces a relatively strong bond, cooling channels can limit the surface area available to bond the backing plate to the sputter target and surface oxides or other impurities can have a significant impact on the bond's strength.